When an electric motor is started by simply connecting the motor directly to the power supply lines (across the line starting), the electric current drawn by the motor can be six times the steady state current once it reaches full operating speed. In addition, the motor torque rises dramatically with this starting technique and such a rise can adversely affect the mechanical components driven by the motor.
In order to reduce the start up current and torque surges, large alternating current motors often are coupled to the electricity supply lines by thyristor switches operated (i.e. triggered or fired) by a controller. When the motor is to be started, the equipment operator applies a starting signal to the motor controller which controls the thyristors to gradually increase the magnitude of the voltage applied to the motor. This is achieved by regulating the period during each half cycle of the alternating supply voltage in which the thyristors are conductive. By gradually increasing the period of conduction, a greater amount of voltage is applied to the motor producing a commensurate increase in motor speed. An inverse technique can be used to decrease the motor speed.
Typically these motor controllers fall into two classes. One of the classes was an analog system as shown in U.S. Pat. No. 3,376,485 in which saw tooth waveforms synchronized to the cyclical supply voltage are thresholded to determine the firing times for the thyristors. By varying the threshold with time, the magnitude of the voltage applied to the motor can be gradually increased or decreased. The other type of motor controllers are digital, often incorporating microprocessors to control the thyristor firing times, as disclosed in U.S. Pat. No. 4,862,052. The microprocessor controllers execute a software program which responds to input signals indicating when the supply voltage and current make zero magnitude crossings. These zero crossing times are used as timing references from which to determine when the thyristors should be fired.